1. Field of the Invention
The present invention is in the general field of water extraction from a gas, such as air. In particular, the present invention provides a method for the extraction of potable water from air in an economical fashion, even in arid environments.
2. Description of Related Art
Obtaining sufficient quantities of drinking water on the battlefield, in disaster situations or in arid environments is a major logistical challenge. Much effort has been aimed at approaches for cleaning liquid water so that it is potable. Such approaches include filtration, distillation, adsorption, ion exchange and reverse osmosis. In general, these techniques remove one or more contaminants from the water including bacteria, viruses, chemicals, and salts. These methods all require the presence of liquid water, such as from a lake or stream.
In many environments, the presence of liquid water of any kind cannot be guaranteed. In contrast, there are always large quantities of water vapor in the air. Even in desert climates, which have high temperature and low humidity, there is a good deal of water in the air since higher temperatures increase the saturation pressure of the water vapor (i.e., the humidity may be low but the actual water partial pressure can be relatively high). Water from air also has applications far beyond the battlefield as a few statistics from the United Nations Educational, Scientific and Cultural Organization (UNESCO) illustrate:                1) Of all water, only 100,000 km3 is fresh surface water, but often is in the wrong area and is polluted;        2) World water consumption in the year 2000 was 2,800 km3;        3) 580,000 km3 of water evaporates and rains/snows every year; and        4) There is 10,000 to 50,000 km3 of water in the air and this water is more uniformly distributed around the world than any other water source.        
As the world's population and per capita water consumption both continue to grow, the availability of fresh drinking water will become more of a problem, especially in arid regions.
In addition to being ubiquitous, the production of liquid water from air potentially has other advantages. For example, the water has already been purified (essentially a solar distillation process) and typically does not suffer from chemical, viral, or bacterial contamination.
The two main approaches for producing water from air have been cooling of the air to condense the water vapor and adsorption of the water vapor directly from the air. Although cooling ambient air and collecting the condensed water is a technically viable approach, the required apparatus is large, heavy and uses large amounts of energy—about 650 W-hr/liter of water depending upon the particular design and the relative humidity of the ambient air. The fundamental problem with cooling air to condense the water vapor is the low concentration of water in the air. Water vapor constitutes only about 1% of the volume of air depending upon the air temperature and humidity. When the vapor is condensed to liquid, it undergoes another volume reduction of about 1000×. Therefore, the production of a liter of liquid water requires the cooling of about 100,000 liters of air.
Adsorption concentrators solve this concentration problem. Although all systems that extract water from air must process large quantities of air to access sufficient water vapor, it is desirable to process the air isothermally and isobarically to reduce overall energy costs, as is the case with an adsorption-based system.
The major problem with current adsorption-based systems is the heat that is generated by the adsorption process and the requirement to regenerate the adsorber by removing water. The most common way to regenerate adsorbents is by thermal swing, i.e., heating the adsorbent to remove water. However, thermal swing requires large temperature changes, precludes rapid cycling and uses a good deal of energy.
U.S. Pat. No. 6,336,957 by Tsymerman discloses an adsorption-based apparatus where thermal swing is used to desorb water from the sorbent material to collect water from air. Ambient air is drawn into a first area of an enclosure which comprises the sorbent material. A second area of the enclosure contains a condenser. After the sorbent material is saturated with water, the first area is hermetically sealed and the sorbent is then heated to cause desorption of the water from the sorbent. A pressure differential is then created between the first area and the second area, whereby water-containing air flows from the first area to the second area and the water is condensed and collected.
U.S. Pat. No. 5,817,167 by DesChamps discloses an adsorption-based apparatus to dehumidify air. The adsorption system comprises at least one heat pipe with desiccant materials applied to the first end of the pipe. During the sorption phase, ambient air is fed through the pipe containing the desiccant and water is adsorbed by the desiccant. The generated heat is transferred from the desiccant to the first end of the pipe, then to the second end of the pipe and finally to a second air stream, thereby maintaining an adiabatic sorption process. During the desorption phase, a heated air stream contacts the second end of the pipe which transfers the heat from the heated air stream to the second end of the pipe and, ultimately, to the desiccant material. The increase in the temperature of the desiccant evaporates water from the surface of the desiccant.
An alternative method to regenerate an adsorbent is pressure swing, i.e., removing water by reducing the pressure over the adsorbent. In this method, the adsorber is connected to a vacuum system that extracts water from the adsorbent and feeds the water vapor to a refrigerated heat exchanger.
U.S. Pat. No. 6,156,102 by Conrad et al. discloses an absorption based apparatus using a pressure and/or thermal swing to collect water from air. A hygroscopic solution is contacted with air wherein the water in the air is absorbed by the hygroscopic solution. The hygroscopic solution and air are separated and the solution is exposed to a temperature swing, pressure swing, or both to release the water from the hygroscopic solution. The use of a membrane to separate water from the hygroscopic solution is also disclosed.
With the use of pressure swing recovery, condensation of the water vapor occurs in the refrigerated heat exchanger and the heat of vaporization is the only load on the refrigerated heat exchanger (i.e., cooling of large volumes of air is not required). With a COP (coefficient of performance) of 2, the overall energy consumption can be reduced to about 350 W-hr/liter of water. However, an energy consumption of about 350 W-hr/liter of water is still over an order of magnitude above the energy required for the production of fresh water using distillation or reverse osmosis of seawater. Therefore, there is a need for a new means of producing potable water from air using a much lower quantity of energy per unit of water extracted. The amount of energy used for a blower to provide sufficient airflow to produce a liter of liquid water from 50% relative humidity air is only about 5 W-hr/liter of water. Thus, if a low energy approach can be used to extract the liquid from the adsorbent, the total energy requirement to extract liquid water from air could approach that of reverse osmosis cleanup of water or be even lower.
U.S. Pat. No. 6,453,684 by Spletzer et al. discloses a method for extracting water from air using non-adiabatic compression. Air is compressed using a piston to a relative humidity of about 1. A temperature gradient is maintained within the piston and water condenses in the cooler region. This condensate is then removed and the remaining air is discharged to the atmosphere. Heat may be removed during the compression step and then added to the expanded, dry gas prior to its release. Spletzer et al. disclose work of 1.3 MJ per kilogram of water extracted at a temperature of 25° C. and a humidity of 40%.
In addition to power consumption, the weight and volume of adsorption-based apparatus are typically too large to consider the system for portable use. Most of the weight and volume are associated with the vapor condensation/refrigeration portion of the apparatus. If the refrigeration/condensation step and the related device can be eliminated, a significant size reduction could be achieved. For the volume of water per weight of the device, current condensation systems achieve about 0.001 liter/hour/kg (e.g., a one kilogram device will process about 1 milliliter of water per hour). Advanced refrigeration/adsorption systems under development could yield 0.002 to 0.005 liter/hr/kg. However, with an improved water recovery system, the system mass could decrease to yield a system rate of 0.2 liter/hr/kg.